12.08.2010 | Author / Editor: E. Pickel and W. Schleyer / Marcel Dröttboom
Extreme ambient conditions require effective and robust sealing arrangements. Sealings in inner bearing arrangements are often tailor-made solutions and vary in terms of design, material, fixation in the pulley, etc. Thus, the focus of this chapter is on sealing of outer bearing arrangements in standard housings. Operating conditions with quite low rotational speed often allow bearings and housings to be 100 percent grease filled when mounted. The grease acts as additional seal.
A common design for split plummer block housings SNL and for non-split housings of series SBD and THD (Fig. 6) is a labyrinth seal. In combination with grease-full-filling and regular periodic/continuous re-lubrication (greasing via labyrinths, amount adapted to pollution rate), entrance of dirt into the bearing can be prevented.
If the ambient conditions require a higher efficient sealing arrangement, the Taconite sealing is used. This combines a labyrinth and a contacting seal. The labyrinth stages can be filled with grease. While re-lubricating a defined grease flow outwards can be realized and entering of dirt is avoided. Based on experiences and depending on sealing design and degree of pollution, sufficient number and position of grease inlets have to be defined. Manual lubrication as well as automatic central lubrication systems can be used.
A rather new option, especially for larger diameters, is the use of sealed spherical roller bearings. These bearings are greased and equipped with a bearing-internal contacting seal. They can be re-lubricated via the annular groove in the outer ring. Surplus grease can escape via the sealings. In combination with the earlier described 100 percent grease filling in the housing, an efficient multiplex sealing arrangement can be realized (Fig. 7). Because the grease inside the bearing and the ‘barrier-grease’ in the housing are separated by the contacting sealing, it is possible to use a biodegradable grease as barrier. Negative environmental impacts and the costs for disposal of the used grease can be reduced.
Depending on the grade of pollution, the use of sealed spherical roller bearings offers different new options. Thus, maintenance-free bearing arrangements or at least with significantly reduced maintenance expenditure can be realized. Furthermore it is possible to reduce the complexity of the housing sealing and/or minimize the quantities for replenishment of the ‘barrier-grease’.
The standard assortment of sealed spherical roller bearings covers in the meantime many bearing sizes and series with inner (bore) diameters up to 400 mm. Especially for larger diameters it is adapted to the commonly used types in open cast mining applications and to the standard housing series (SNL, SBD, THD etc.). For special applications also extra large sealed spherical roller bearings are in field already. The patented sealing design for those allows higher values of misalignment.?
[1] SKF: Main catalogue. Brochure 6000 EN (2007).
[2] SKF: SNL 30, SNL 31 and SNL 32 plummer block housings solve the housing problems. Brochure 6101 E (2008).
[3] SKF: SNL plummer block housings solve the housing problems. Brochure 6112 E (2008).
[4] SKF: Interactive Engineering Catalogue. www.iec.skf.com.
Edgar Pickel and Werner Schleyer, SKF Gmbh, Germany
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